The present invention relates to a roll-type pressure fixing and developing apparatus in which ink containing sheet material to be treated is passed through a high pressure nip defined by a pair of rollers and is also directed to a roller for such an apparatus.
It is known to pass sheet media on which an image is to be fixed or developed through a high pressure nip defined by a pair of rollers. Commonly, the outer ends of at least one of the two rollers is loaded with a pressure applying load applied in a direction normal to the axis of the roller. This load is also applied in a direction which biases the rollers together to form the nip. Application of a load in this manner results in bending moments in the loaded roller. The bending moments cause the roller to deflect or bow in the center such that there is a reduced or minimum fixing or developing pressure at the center of the nip. This deflection at the center of the roller increases when a sheet of media is inserted in the nip. As a result, uneven fixing of toners and ink to the media occurs. Higher pressures than necessary to fix toners and ink are then required at the ends of the nip to assure adequate pressure fixing at the center of the nip.
There are also devices in the prior art in which the rollers are skewed, that is, their longitudinal axes are supported out of parallel with one another, to compensate for the deflection of the fixing rollers. Skewing the rollers allows the ends of the rollers to wrap around each other as they deflect under load, resulting in more uniform pressure along the nip. However, skewing the rollers also results in forces which act on the media in a direction substantially perpendicular to the path the media travels. These lateral forces tend to crease or curl the media during passage through the nip. Also, such an apparatus is generally limited to pressure fixing at one pressure value. For example, higher pressure causes greater deflection in the rollers, which requires a greater skewing angle to avoid non-uniformity along the nip.
Other expedients have been introduced in an attempt to overcome the problem of deflection of fixing rollers upon the application of force to the ends of the rollers. Large diameter fixing rollers reduce, but do not eliminate, the deflection. However, large diameter fixing rollers add weight, cost and bulk to the apparatus. A backup roller or rollers in pressure contact with the pressure fixing and developing rollers has also been used to urge the pressure fixing rollers together along the nip. However, such backup roller systems require additional space for the backup rollers and also require additional components in comparison to a two-roller system.
Another method suggested in the prior art is the use of a roller which is crowned at the center to compensate for the deflection due to loading. However, crowned rollers have a faster surface velocity at their center than at their ends. This differential in surface velocity contributes to wrinkling of the media and limits the versatility of these devices in handling various types of substrates.
As a more specific example of the prior art, U.S. Pat. No. 4,363,862 of Giorgini discloses an apparatus for fixing toner powder images on sheet material. In Giorgini, a non-compliant pressure roller and compliant back up roller are supported with skewed longitudinal axes. The pressure roller is a rigid non-compliant material, such as steel, with a non-compliant outer layer having an irregular surface comprising a plurality of randomly sized dome projections. Chrome is one example of this surface material. The backup roller is described as having a sheath of a compliant material over a central core. Organic polymeric substances are mentioned as suitable for the sheath, with nylon 6/6, glass filled nylon, hard rubber and acetal resins being specifically mentioned. The core of this backup roller is described as being of a rigid non-compliant material.
U.S. Pat. No. 4,768,050 of Beery discloses a pair of pressure rollers used in conjunction with the "Mead" imaging process in which photosensitive micro capsules are ruptured by the rollers to provide the image. This apparatus includes a first roller having a hollow shell with an axially central part defining a loading region. A loading shaft extends through the shell and is joined to the shell only at the central loading region. The interior surface of the shell is partially tapered, and is described as controlling the deflection of the shell portion of the roller when the roller is loaded at its ends. The rollers define a working zone and are relieved at their ends to provide non-working regions. With this design, due to the excessive pressures at the ends of the rollers, the working zone must be less than the width of the rollers, as otherwise sheet material passing through the rollers can be damaged. Therefore, this apparatus lacks versatility in being able to effectively treat sheet material of varying widths.
U.S. Pat. No. 4,356,764 of Haugen discloses a pair of rollers which each include a central core of a uniform diameter and an outer hollow shell. Pressure transfer rollers support the shells on their respective cores with the transfer rollers engaging the supported shells at four spaced locations. Because the shell is supported at discrete locations by the transfer rollers, a less than uniform pressure would be expected along the nip. This is particularly expected to be the case when one of the outer cylindrical shells is made of an elastic material, such as rubber of about 35-60 durometer, as mentioned in one embodiment of this apparatus.
The use of pressure fixing rolls for fusing or spreading solidified hot melt ink on print media is also known. Japanese Patent Specification No. 60-18,351 of Moriguchi, et al. and U.S. Pat. No. 4,745,420 of Gerstenmaier are two examples of these types of devices. The construction of pressure rollers is not described in these two patents. Also, the Gerstenmaier patent uses an eccentric for varying the pressure applied to the rollers. Other examples of prior art image fixing apparatus including rollers are described in U.S. Pat. Nos. 3,293,059 of Stowell, 3,566,076 of Fantuzzo and 4,568,949 of Muranaka.
Prior art pressure fixing rolls known to the inventor suffer from problems when treating media of widely varying thickness types and/or widths.
Although a number of roll-type pressure developing and fixing devices are known, a need exists for such an apparatus, and for rollers for such an apparatus, which is capable of overcoming these and other disadvantages of the prior art.